Herakles fights a Kyknos in the Hesiodic Shield (“Aspis”; “Scutum”). Achilles fought one too according to some accounts. N.B. “kyknos” means “swan” in Greek, whence English “cygnet”.
Athenaios, Deipnosophists, 9.393de
“Hegesianax the Alexandrian, who composed the Troika attributed to Kephalion, says that Kyknos who fought in single combat against Achilles was raised in Leukophrys by a bird of the same name.”
ὁ δὲ τὰ Κεφαλίωνος ἐπιγραφόμενα Τρωϊκὰ συνθεὶς Ἡγησιάναξ ὁ Ἀλεξανδρεὺς καὶ τὸν Ἀχιλλεῖ μονομαχήσαντα Κύκνον φησὶ τραφῆναι ἐν Λευκόφρυι πρὸς τοῦ ὁμωνύμου ὄρνιθος.
Scholia to Lykophron 232
“Lykophron lies about this too. For Kyknos was not killed because he was struck on his shoulders, but in his head. For they say that he was invulnerable in the rest of his body except for his head. This is because Kyknos was conspicuous as a soldier and looked so very beautiful among the enemy that it was not expected that he would be wounded in the stories because he was just so unwoundable. But when he was killed because Achilles struck him in the head, they said he was invulnerable except for only the head”
πληγέντα τοὺς κλεῖδας καὶ τοὺς ὤμους. καὶ τοῦτο ψεύδεται ὁ Λυκόφρων· οὐ γὰρ κατὰ τοὺς ὤμους, ἀλλὰ τὴν κεφαλὴν πληγεὶς ὁ Κύκνος ἀνῃρέθη. φασὶ γὰρ ὅτι ἄτρωτος ἦν τὸ λοιπὸν σῶμα πλὴν τῆς κεφαλῆς. τοῦτο δὲ μῦθός ἐστι· περιδέξιος γὰρ ὢν στρατιώτης ὁ Κύκνος καὶ κάλ-λιστα σκεπόμενος ἐν πολέμοις ὡς μηδέποτε τρωθῆ*ναι* ὑποπέπτωκε τοῖς μύθοις ὅτι ἄτρωτός ἐστιν. ἐπεὶ δὲ πληγεὶς ὑπ’ ᾿Αχιλέως τὴν κεφαλὴν ἀνῃρέθη, ἔφασαν ἄτρωτον εἶναι πλὴν μόνης τῆς κεφαλῆς. T καὶ δὴ διπλᾶ· Κύκνος